The exhaust gases of combustion engines, which are operated with an air ratio of lambda=1, also Otto combustion engines are effectively purified with the aid of so-called three-way catalytic converters. Thereby in particular nitrous gases, hydrocarbons and carbon monoxide are removed from the exhaust gas. The exhaust gases of combustion engines, which work with an air ratio of lambda >1, thus for example Diesel combustion engines or Otto combustion engines in lean operation, can be purified only partially with such catalyzers. Hereby only the hydrocarbons and the carbon monoxide are mostly degraded, while nitrous gases cannot be noticeably reduced due to the high oxygen content in the exhaust gas.
To enable also a purification of nitrous gases in exhaust gases of such combustion engines the so-called selective catalytic reduction is known (SCR-procedure). Hereby ammoniac is often used as the reducing agent, which is created at the SCR-procedure by a urea-hydrolysis. The urea that is carried along in a watery solution serves as ammoniac carrier and is converted to ammoniac with the aid of a metering system by hydrolysis-catalyzer, which then reduces nitrous gases in the SCR-catalyzer. The watery urea-solution, subsequently called “reducing agent”, is stored in a tank, subsequently also termed as HWL tank.
A procedure and a device for metering a corresponding reagent or reducing agent for purifying the exhaust gas of combustion engines is for example known from DE 10 2004 021 372 A1.
The watery urea-water solution, which is stored in the HWL-tank, freezes there at a temperature of −11° C. In that case the system start and therefore the NOx-reduction is delayed depending on the exact tank temperature according to the maximum melting time that is set by the environmental authorities in order to melt a minimum amount HWL in this time. In order to determine the temperature in the HWL tank, a temperature sensor is built into the HWL tank. This tank temperature sensor has to be controlled as exhaust gas relevant component according to the regulations of the On-Board diagnosis II (OBD II). The OBD II requires that exhaust gas relevant components fulfill the following requirements:                SCR-signal range check: continuous electric control of exhaust gas relevant components;        rationality faults: function for a reasonability check, in order to detect, whether the sensor value is moving in a reasonable range and whether it provides a reasonable behavior;        manipulation detection: a simple manipulation of the exhaust gas relevant components has to be detectable.        
In order to realize this function at a temperature sensor, which is built into a HWL tank, which means in order to undertake a reliable reasonability check of a temperature sensor, a second temperature sensor has to be provided for example, with whose aid the first temperature sensor is checked. But the construction of a second temperature sensor is very costly.
The task of the invention is therefore to provide a procedure for a reasonability check of the signal of a temperature sensor that is arranged in a HWL tank, which works without a second temperature sensor and can therefore be technically simply and affordably realized.